A molded case air circuit breaker is designed and tested to safely interrupt a certain level of fault. However, if the fault is of a significant magnitude, gases in the immediate vicinity of the circuit breaker will be ionized sufficiently to allow an arc to occur inside the circuit breaker, should it be reclosed on the fault before these gases have dissipated. This arcing causes sharp gas pressure increases inside the breaker, and the gases are then forced out all of the circuit breaker's openings. Some of the gas will surround the breaker's terminals and nearby bus bars. An arc between phases (external to the circuit breaker) can result and will persist until something clears the fault. Catastrophic damage often follows. Indeed, a major cause of electrical accidents is the failure of circuit breakers.
If the operator is forced to wait three minutes before reclosing the breaker, this phenomenon and contingent damage can be prevented. (Underwriters Laboratories allows a molded case breaker three minutes between fault tests and the second fault must be cleared.)
In mining applications where trailing cables are used extensively, most circuit breaker tripping is caused by the cable being crushed or partially cut into by mobile equipment. The fault most often involves one or two of the conductors and the ground conductor. Federal law requires that a continuously monitored and fail-save grounding system be provided for mining electrical equipment, where resistor(s) must be used to limit the fault current which may enter the ground system to 25 Amps. Reclosing the breaker eight or even ten times on a fault of this low magnitude will not cause this ionized gas and subsequent arcing problem. An operator cannot always determine the cause of the fault, and knowing the cause does not prevent the operator from unwittingly reclosing the breaker.
A common scenario is seen when this problem occurs: The mining equipment operator is usually working away from the mine power distribution center when a fault occurs and the circuit breaker trips. He then must stop working and investigate the cause of the power being shutoff, and proceeds to the power distribution center. He sees that the breaker has tripped and recloses it. Generally, it has taken longer than three minutes to get to there, so that if a fault is still there the circuit breaker will simply trip again. The operator then may attempt to reclose the breaker, unaware (uneducated re: circuit breakers) that it is best to wait several minutes before reclosing.
Hence, a device is needed which:
responds only to abnormally high currents if properly applied PA1 prevents the circuit breaker from being reclosed for a predetermined period of time PA1 provides (optional) means, during the elapse of this time period, to indicate that the device has operated
In a three phase electrical system, a fault which occurs may be one of the following: single line-to-ground fault, line-to-line fault, double line-to-ground fault, or three-phase symmetrical fault. The targets of the circuit breaker protection apparatus of this invention are line-to-line and three-phase symmetrical fault types.
When a line-to-line fault occurs, the current in the third line which is not involved falls to zero. The current in the two affected lines is equal in magnitude, and opposite in direction. When a three phase symmetrical fault occurs, the currents in all three are equal in magnitude, and each is 120 degrees out of phase from the other two. Therefore, at least two phases must be monitored, since the current in the unfaulted phase falls to zero during a line-to-line fault.
A wide variety of circuit breakers is available, however, no known device provides the capability of preventing the circuit from being reclosed for a period of time. The "circuit breaker saver" of the present invention satisfies the requirements very effectively, and also requires only a small enclosure space--an attractive feature where the enclosure is already very crowded with the various control devices comprising the circuit.